1. Field of the Invention
The present invention relates to handling devices for fluids, including but not limited to gases or liquids, and specifically to the control of small quantities of fluids and to devices for performing measurements on small quantities of gases or liquids.
2. Description of the Background
One common way to determine gas sorption properties of a material is to measure changes in pressure in a calibrated volume as gas is sorbed by a test sample of the material. The quantity of gas sorbed in each measurement is found from the equation of state of the gas. It is necessary to know three parameters to determine the quantity of gas sorbed: the pressure, temperature and volume of the gas. At constant volume and temperature, the quantity of gas sorbed is determined by measuring the change in pressure. From a known sample mass it is then possible to determine the mass concentration of gas that has been sorbed or desorbed by the sample. If the composition of the sample is well known, then the stoichiometry of gas in the sample may also be determined from the measured concentration.
Thus, for example, the pressure-composition-temperature characteristic curve (a “PCT” curve) of a sample may be obtained by dosing the sample with small “aliquots” of gas from a small volume or desorbed into a small volume such that only a small fraction of gas is sorbed or desorbed at one time. A sorption PCT curve is measured by increasing the pressure in each aliquot of gas applied to the sample in a step-wise fashion. Similarly, a desorption PCT diagram is measured by decreasing the pressure in a step-wise fashion, in the small volume into which the sample is desorbed. The conventional apparatus for performing such measurements is referred to as a Sieverts' device.
Typically, a gas dosing apparatus or Sieverts device involves dosing gas from a known volume using standard orifice and seal valves or on/off valves. Common examples of the types of valves are diaphragm valves or needle valves. These valves may be automated using pneumatic plungers or electrical solenoid mechanisms. One problem with these types of valves is that the diaphragm or stem moves in a lateral motion to open or close the valve, causing an internal volume change in the system as a whole. Volume changes on the order of 0.05 milliliters, for example, may cause significant measurement error if not taken into account when calculating the mass balance of a gas dosing system. This mass balance is critical when making specific types of measurements such quantifying the amount of gas sorption to or from a sample.
There are many problems and issues with conventional Sieverts' devices and with other prior art methods and devices for measuring sorption. Conventional devices must be operated for long periods of time taking many measurements, such as are required for generating a PCT curve. This may involve large numbers of repetitive operations, such as delivering aliquots in a PCT measurement, or switching between sorption and desorption in cycle-life measurements. It is important to obtain evenly spaced data points of sorption/desorption measurement along PCT curve. Without a detailed and even distribution of data points over the entire pressure range, the PCT curve will not be well resolved and certain portions may not be observed at all. Thus, for example, changes in the equilibrium plateau pressure identified with hydride phase transitions could be missed entirely.
Variations in air temperature in a room in which measurements are performed, or variations in the gas temperature throughout a measurement device, can produce significant errors in measuring the quantity of gases sorption or desorption from a material. Even if the surrounding air temperature is measured and introduced into the equation of state, the time lag between changes in the temperature of the surrounding air and the temperature of the gases in a Sieverts apparatus can be significant enough that the data can not be sufficiently corrected.
Small samples, such as those of 1 gram or less, and/or samples that adsorb or absorb only small quantities of gas, for example 50 milliliters STP or less, are difficult to investigate using typical volumetric devices that often have calibrated volumes and piping with volumes on the order of 10 milliliters or more. Thus, for example, standard on/off valves have internal volumes greater than 1 milliliter, especially when designed to operate at pressures greater than 30 atmospheres. However, to be able to measure gas sorption on very small samples using the Sieverts method, very small dosing volumes are required. For example, a pressure concentration temperature isotherm measurement on a 20 mg sample with 1 weight percent gas uptake under ambient conditions would require a dosing volume on the order of 0.5 milliliters. This is much smaller that the standard valves' internal volume, without even considering the volume of any tubing, pressure transducers, or fittings.
Manual orifice-and-seat-type valves found in prior art devices are subject to inconsistent operator behavior. In particular, operators have a tendency to over-tighten valves during high-pressure experiments, resulting in valve seat damage causing the valve to leak. Manual systems have been known to operate for only a few experiments before the valves leak to the point that the data is seriously compromised.
Thus there is a need in the art for a method and apparatus that permits the accurate measurement of sorption characteristics using small quantities of sorption materials. The apparatus should preferably be easily automated, capable of operation at constant or measurable temperatures, and having repeatable volumes. There is also a need in the art for a dosing valve and other system components have very small internal volumes.